1. Field of the Invention
The present invention relates to fiber reinforced, castable matrices, and more particularly to castable matrices, such as cement products, reinforced with chemically treated steel fibers to form an admixture having improved creep resistance, and exhibiting improved mechanical properties.
2. Description of the Prior Art
The use of short length steel fibers randomly dispersed in concrete products has been found to improve the compressive-impact, flexural and crack propagation properties in the reinforced product. Numerous applications for fiber-reinforced cement products are known, but they are principally for controlling crack propagation, and not for continuous load-bearing structural purposes.
Many efforts have been made to adapt fibers for use in structural applications of this type, but all attempts involve alteration of the shape of the fibers. One example of this is the deformation of the fibers for the purpose of increasing their tensile-shear bond to concrete and other cement products. Although the pull-out resistance of such deformed fibers increases, nonetheless, it has been found that the fibers pull out while under constant load. Thus, no evidence has yet been found to support a conclusion that fiber reinforcement has a significant effect on the creep behavior of portland cement mortar (see State-of-the-Art Report on Fiber Reinforced Concrete, Report No. ACI 544.1R-82). Unlike conventional reinforced concrete, maximum load is controlled primarily by fibers gradually pulling out, and the stress in the fiber at the ultimate load is substantially less than the yield stress of the fiber itself.
In present state-of-the-art reinforced concrete technology, the bond between steel fibers and concrete is considered to be derived from two sources.
First, when steel reinforcing elements such as fibers, wires or rebars are used, it has been determined that a frictional bond exists between the concrete and the steel elements which increases as the concrete ages, and which is due to the shrinkage of the concrete around the steel elements.
Second, when steel reinforcing elements such as bars having deformations (i.e., lugs or protrusions) are used, it has been found that longitudinal movement of the steel bars in the concrete is inhibited.
In no case has evidence been found of a chemical bond between the steel reinforcing elements and the concrete. In fact, no traces of hardened cement paste have been found on the surfaces of the conventionally used steel reinforcing elements after debonding.
In "Limit State Design of Structural Concrete" by Regan and Yu, an approximate relationship for shear stress and relevant factors is described, and the following formula is provided: EQU shear stress=(K.sub.1).multidot.(K.sub.2).multidot.(f.sub.cr)
where K.sub.1 is a coefficient depending on the bar surface, K.sub.2 is a coefficient depending on the position of the bar and f.sub.cr is the 28 day compressive strength.
K.sub.1 ranges from 0.04-0.16 depending on whether the surface is heavily pitted, moderately pitted or is rolled. K.sub.2 is 1.0 for vertical bars and 0.33-0.5 for horizontal bars. Chemical bond is not present as a relevant factor.
Consideration has been given to precoating steel fibers with plastic materials to increase the interfacial shear strength between the fibers and concrete (see U.S. Pat. No. 3,650,785 to Ball et al.). The materials proposed were liquid type organic coatings such as epoxy resin adhesives, organic phosphate resin and other organic resin mixtures.
However, the system of Ball et al. has exhibited several serious limitations. Most notable is the effect of interposing one of above-identified materials, which typically has a Young's modulus of elasticity (E) of about 350,000 psi, between the steel (E=30,000,000 psi) and the concrete (2,500,000 psi). In such situations, the steel has effectively been prevented from reinforcing the concrete. Moreover, interfacial shear stress applied to such a system tends to cause a large strain in the coating and to thereby cause failure in the concrete due to lack of transfer of support from the steel.
Clearly there is a need for a bonding agent with a modulus of elasticity (E) close to that of steel. Moreover, whereas liquid adhesives such as those disclosed by Ball et al. bond well to hardened concrete, concrete paste does not bond to cured or hardened plastics.